Of the two major classes of structural macromolecules in the corneal stroma, the proteoglycans are less well described. However, both collagen and proteoglycans are crucial to the development and maintenance of optical clarity of the cornea. Two major proteoglycans synthesized by rhesus monkey corneas in culture have been characterized and seem to be similar to those synthesized under the same conditions by human material. Alterations in the normal composition of proteoglycans may accompany or even be the mechanism for certain visually disabling corneal diseases.